The present invention relates to a fuel supply system suitable for supplying the fuel to an internal combustion engine, or more in particular to a fuel supply system having the functions of metering and atomizing the fuel.
In the conventional field of internal combustion engines for automobiles, for example, the fuel is supplied by way of an injection valve of magnetic coil type. In a fuel supply system of this type, as well known, a valve held by a spring under the electromagnetic force generated in a coil is pulled up to meter the fuel while injecting the same. The conventional injection valve of magnetic coil type injects the fuel with a particle size of approximately 300 .mu.m. The fuel of this particle size partially fails to ride the gas stream in the intake air system and attaches onto the wall of the intake manifold without being transported. The fuel thus deposited flows into the cylinder in the form of a liquid film with the occasional result that the air-fuel ratio deviates from a target value.
In conventionally-suggested means of atomizing the fuel, a fuel injection valve of magnetic coil type is combined with a vibrator for providing an ultrasonic vibration to cause the injected fuel to bombard or contact a vibrating member, thereby promoting the atomization thereof. Such a fuel supply system which operates to atomize the fuel by using a vibrating member is disclosed, for example, in JP-A-53-140417 and JP-A-54-47926.
As described above, in the prior art, the fuel is metered and atomized by a combination of a fuel injection valve of magnetic coil type and a vibrating member. Such a fuel supply system, however, is characterized by an increased number of parts, which makes the system bulky. Another problem of the fuel injection valve of magnetic coil type is the long time required before the plunger attraction and the pull-up operation of the valve body are effected following the energization of the magnetic coil. Especially when the engine is running at high speed with injection signals produced at intervals of several milliseconds, the valve is liable to open at delayed timings resulting in a reduced amount of fuel supply. In order to obviate this problem, a higher response speed is demanded.
In response to this demand, U.S. Pat. No. 2855244, JP-A-52-25926 and JP-A-54-10951 proposes a fuel supply system in which the fuel is led to an injection port formed at the forward end of a vibration-amplifying horn excited by a piezoelectric device through a fuel passage formed in the horn, and the fuel is supplied therefrom in atomized form into the intake manifold.
These systems have a ball valve mounted at the horn injection port thereof and operate to apply a pulse drive signal of a frequency equal to the resonant frequency of the horn to the horndriving piezoelectric device thereby to open the injection port for fuel injection.
The aforementioned prior-art systems thus control the fuel flow rate of a fuel supply system by changing the frequency of a pulse drive signal applied to a piezoelectric device for driving a horn.
These prior-art fuel supply systems, however, have a disadvantage that the quantitatively meterable range of the flow rate of the fuel injected in resonance with the horn is comparatively narrow. Specifically, when the engine speed exceeds 2,000 to 3,000 r.p.m., an excessive fuel rate makes it impossible for the system to atomize the fuel any more.
Further, in view of the fact that the horn is resonated only during the fuel injection time, the fuel injected immediately before the ball valve closes fails to be atomized and is liable to contain coarse fuel particles.